Automatic equalization of video signals

ABSTRACT

A video compensation system for analog video transmission is described. The compensation system is employed in an analog video switching circuit such that each time a conductive path is switched, the system automatically tests the new switch path for a new compensation value. The compensation value is determined by measuring the response of the new path to a set of tones that are applied to the conductive path, the response to which is measured against a table of responses previously recorded. The measured responses are compared to the recorded responses to determine an appropriate compensation control voltage, which is applied to an equalizer system. In an alternative embodiment, the skew compensations also provided between red, green, and blue twisted pair lines in the cables by performing comparative analysis between corresponding pairs of the red, green, and blue signals.

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This invention relates to analog video switching. Thisapplication claims the priority of U.S. Application No. 60/356,706, theentire contents of which are incorporated herein by reference.

BACKGROUND AND SUMMARY OF THE INVENTION

[0002] The present invention has application within the environment ofanalog video extenders. In modern environments, one embodiment isincorporated into keyboard, video mouse (KVM) switches that connectmultiple user workstations (such as keyboards, mice, monitors, etc.)with selected ones of multiple different servers. Analog KVM switchesroute video between the workstations and the servers in the analogdomain and provide for high bandwidth real-time video and multimediatransmissions. One such analog KVM switch is the analog matrix switch(AMS) developed and sold by Avocent Corporation of Huntsville, Ala. Anexample analog switch architecture is shown in FIG. 1.

[0003] In FIG. 1, the system is made up of servers 10-15, computerinterface pods (CIP) 16-21, analog matrix switches (AMS) 22-24, analoguser pods (AUP) 31-35, and user workstations 36-40. Also shown in FIG. 1are a hub 26 communicating with switch or router 27, communicating withcorporate LAN 28, and communicating with a matrix system administrator(MSA) 29 operating on a computer 30. Also shown in FIG. 1 is a localcomputer 41 communicating with workstation 40 through CIP 41 and AUP 35.

[0004] The example in FIG. 1 is but one example embodiment of how theAMS system can be arranged into a distributed architecture, and manyother architectures (both of simpler and more complex arrangement) willbe understood to the artisan upon review of FIG. 1. Further, the exampleof FIG. 1 is described simply to give an example context into which thepresent invention may have application and in no way is intended tolimit the broad aspects of the present invention.

[0005] The system in FIG. 1 provides command, control, and switching ofKVM signals between servers 10-15 and workstations 36-40, as well ascontrolled by MSA application 29 of the system as a whole. The AMSsystem of FIG. 1 operates independently of software applications on theservers or workstations. In essence, the system is responsible forestablishing connectivity paths between users at the workstations 36-40and servers 10-15, switching and routing of KVM signals throughout thesystem, user authentication, and software upgrades at the unit endsystem level as directed by the MSA 29.

[0006] As background, the AMS 22-24 connects to servers 10-15 via CIPs16-21. CIPs convert the native KVM connections to proprietary longdistance signals and serves as the interface between an individualserver and the KVM matrix system. The connections between CIP and theAMS units can be by industry standard UTPlus cabling, as are all otherconnections between the system elements.

[0007] On the workstation side, users are connected to the AMS 22-24 viaAUPs 31-35. AUPs are a desktop design with a universal power supply andcan provide peripheral support for a variety of different workstationtypes, such as PS/2, Sun, etc. As shown in FIG. 1, the AUP may bedirectly connected via UTPlus cable to one-to-four AMS or CIP modules.The AUPs can be in a mix and match configuration, such as AUP 34, whichcommunicates with AMS 25 and CIP 21.

[0008] Because structure of the system employing the present inventionis not a critical aspect of the present invention, the example of FIG. 1is relevant only for its illustration of transmission of analog videosignals between the component servers, CIPs, AMSs, AUPs, and theworkstation.

[0009] In an example of FIG. 1, the AMS 22-25 connects to servers 10-15via CIP 16-21 to convert KVM signals from native connectors and cablingto long line (long distance) communication protocols. The physicalconnection between the CIPs and the AMSs is via UTPlus cable. On theother end, the AMSs 22-24 connect to AUPs 31-35 also by UTPlus cable.The maximum distance between CIPs and AUPs is dictated by thedegradation that occurs in the analog video signal over long distances.Distances between CIPs and AUPs of about 300 meters are difficult toobtain for high bandwidth, high quality video having presentcompensation schemes. The principle purpose of the AMS is to switchvideo (and data) between any of its inputs and outputs, therebyconnecting selected workstations 36-40 with selected computers 10-15.

[0010] The analog user pod (AUP) is the main user console interfacecomponent of the AMS system. The AUP 31-35 provides KVM connectivitybetween the user console and either AMSs 22-25 or CIPs 16-21. Because avideo degradation occurs in the cabling, the present invention relatesto methods for correcting the video distortion caused by signal lossesin the video transmission. In one example, such correction occurs in theAUP 31-35 and is carried out during each switch from a user to a server.The AUP also provides on-screen display menu-based technology to permitusers to select new computers for connection via an on-screen menu. Eachtime the user employs the OSD menu to select a new computer, the videocorrection functions can be performed by the AUP to permit fullycompensated video to be provided to the user shortly after the switchrequest is initiated.

[0011] On the other end, the computer interface pod (CIP) convertscomputer KVM signals into a format that can be transmitted down UTPluscable to the AMS or AUP. All of the UTPlus cable described herein isusually four-pair unshielded, twisted pair cable that is rated categoryfive or better. Other alternative cables are, of course, employable in asystem that also employs the compensation systems of the presentinvention. Each CIP employs one KVM computer port, using nativeconnectors for servers 10-15.

[0012] The matrix switch administrator (MSA) 29 is a client software oncomputer 30 that allows the administrator of the analog matrix system toeasily configure, monitor and maintain the system from a remote computer30 on an attached LAN 28 or connected via a cross-over cable. The MSA 29allows the administrator to perform functions such as user settings,server settings, system monitoring, system administration, systemlogging, etc. The MSA 29 may also perform port status, event logging,trace routing, etc., via it's network port.

[0013] As previously described, in the system of FIG. 1, and otheranalog matrix switching systems, analog video is degraded as it travelsalong cables. The present invention compensates for video distortion onindustry standard cabling (such as CAT5, CAT5e and CAT-6 (includinggigaflex), and any other standard cabling) to provide high quality videoup to 1,000 feet away from servers 10-15. The compensation isaccomplished by three compensation features, used independently or inany combination. The first is automatic adaptive cable equalization inwhich the system automatically corrects for frequency dependentattenuation each time a valid KVM path is selected. The second isautomatic adaptive cable de-skew compensation that automatically detectsand corrects for inter-pair delay skew that is inherent in, for example,CAT5 style cables. The third is compensation obtained by reducing datalink pair cross talk in CAT5 RJ45 connectors.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a schematic representation of an example matrixswitching system into which the present invention may have application;

[0015]FIG. 2 is a schematic diagram of example video equalizationcircuits;

[0016]FIG. 3 is a schematic representation of example cable measurementcircuits;

[0017]FIG. 4 is an example data chart of laboratory measurements of acable in accordance with an example embodiment of the present invention;

[0018]FIG. 5 is an example measurement map of a cable in an exampleembodiment of the present invention;

[0019]FIG. 6 is an example equalization control voltage chart inaccordance with an example embodiment of the present invention;

[0020]FIG. 7 is an example actively controlled delay line circuit;

[0021]FIG. 8 is an example embodiment of de-skew compensation circuitry;and

[0022]FIG. 9 is an example flow chart of an equalization system.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

[0023] In the preferred embodiment of the present invention, the matrixswitching system performs video compensation on-the-fly as switchingoccurs between the workstations 31-35 and the servers 10-15. That is, aseach new connection occurs between a workstation and a server, a newpath of, for example, CAT5 cable is employed that may have its ownunique video degradation characteristics for which compensation isdesired. A single compensation system for a workstation and for a servermay not successfully compensate when the workstation is communicatingwith a different server, or vice a versa. In accordance with thepreferred embodiment, each time a switching operation occurs, new videocompensation is applied to the video signal in order to compensate forthe unique characteristics of the new connectivity path.

[0024] Automatic Adaptive Cable Equalization.

[0025] Equalization is the compensation for normal frequency dependenterror related to the skin effect on cables. To equalize, one must applyan opposite frequency response curve to boost video signals receivedfrom the cables at the reception end. A problem occurs in determininghow much frequency response to apply, and in ensuring that the frequencyresponse compensation is done transparently to the user. The prior artprovided amplitude compensation with frequency dependent gain stages,with the amplitude set for an arbitrary length of cable. The preferredembodiment provides equalization that is independent of cable type andcable length, such that the equalization can be adapted for variouscable types and lengths.

[0026] In the preferred embodiment, a cable is swept with a set of tonesand the response to the tones recorded. In a preferred example, sixtones are employed, although other numbers can be employed for greateror less sensitivity. Prior to implementation, the amplitude of each toneover the cable (for various cable lengths) is measured and a coefficientgiving the frequency response of the cable on each length is determined.After implementation, software routines at the end of the cablereceiving the video components generate control voltages in accordancewith the coefficients determined. Importantly, all of this can be donein about a tenth of a second, using CAT5 cable.

[0027] For implementation of the software, preferably in a laboratory, alist of coefficients for ten foot segments (or more or less, dependingon sensitivity desired) up to 1,000 feet for the various tones iscreated and stored into a memory table. Examples of those coefficientsare shown in the Appendix of U.S. Application No. 60/356,706, which isincorporated by reference and, for brevity will not be repeated herein.For illustration purposes, one of such charts is shown in FIG. 4, for anexample 90 ft. cable length.

[0028] Presently, CAT5 cables are traditionally used for thetransmission of video signals, which cable is 24 AWG and therefore theskin effect (which is based primarily on the diameter of a wire) issubstantially the same for all CAT5 transmission systems. Thus, thecoefficients recorded in the laboratory for CAT5 cable should accuratelyrepresent CAT5 cables, provided the 24 AWG dimension is beingmaintained. If other dimensions are desired, of course other laboratorymeasurements can be employed to create different tables, which can beused instead of, or in addition to, 24 AWG cable tables.

[0029] The table of ten foot interval coefficients are recorded into theAUP 31-35 of a KVM switch system. Next, a second table (FIG. 5) isprepared (again in a lab) which identifies, for fifty foot cableincrements, control voltages versus distance needed to correct videosignals on the cable. That example chart of FIG. 5 is shown at theindicated fifty foot increments. The incremental values are notcritical—they can be more or less than 50 feet for differentsensitivities.

[0030] In practice, the computer interface pod (IP 16-21) sends thetones (preferably the six tones) down each of the color component CAT5cables and the frequency and amplitude of the tones are measured at theAUP 31-35. These measured coefficients are compared to the ten footcoefficient versus distance table (FIG. 4, in part) to determine a bestfit, which identifies a “distance” identifier (even though the distancemay not necessarily exactly equate with the true distance of the cable).The distance is then compared to the fifty foot control voltages versusdistance table to determine an appropriate control voltage for the“distance” provided by the first map. The result is a control voltagethat can be applied to the CAT5 component cable to provide frequencycompensation.

[0031] The control voltages are applied to DC(V_(DC)), LO(V_(L)),MD(V_(m)), and HI(V_(H)) stage amplifiers acting on the color componentsas shown in FIG. 2. An example of the control voltages for an examplecable is shown in FIG. 6.

[0032] The preferred apparatus and methodology can accommodate variouswire types, is independent of twist rates, is adaptive to differentcable types and lengths, and is not distance dependent. Each of theseare substantial advantages over the prior art.

[0033] The preferred product can send the tones from the CIP to the AUP(where the compensation software and tables are loaded) in about 100milliseconds, which will typically be transparent to a user if the useris the only user attached to the respective computer through the matrixswitch AMS. If, however, the time is objectionably long (for example,resulting in flicker, etc.) or if a second user is simultaneouslyconnecting to the computer (which may result in a flicker on the seconduser's monitor), the tones can be sent during several consecutiveblanking intervals, for example during six consecutive blanking periods.In the latter case, during each of the six blanking periods, one of thesix tones can be sent down the CAT5 cable to provide the tone test.

[0034] When a user selects a server from the OSD in the AUP 31-35, arouting protocol then determines the appropriate KVM path to theselected server. This path is then opened by the AMS. A short time latera tone sequence request is sent to the CIP that is attached to theselected server. The CIP then sends a number of tones between 32.35 kHzand 48 MHz simultaneously on all three guns (R, G, B).

[0035] The corresponding received amplitude for each tone is processedby a Log Amp 55 whose resultant output is then digitized by a fast A/Dconverter 56. The use of a Log Amp results in a log slope of 24 mV foreach dB of signal change. The ability to compress signals of widedynamic range avoids the use of gain or range switching at the front endand improves overall measurement speed.

[0036] The values returned by the measurement system are a decibelrepresentation of the frequency response of the link at discrete points.

[0037] The returned values are then compared with the reference cablemapping taken at 10 ft. intervals for the cable type. As previouslydescribed, that map resides within the AUP memory.

[0038] Associated with this reference cable mapping are the appropriateequalization control voltages that were determined empirically at 50 ft.intervals for the same reference CAT5 cable. In one example embodiment,control voltages for any 10 ft. segment between 50 ft. lengths aredetermined by piecewise linearization (i.e., it is assumed that theequalizer control voltages follow an approximate linear slope betweenany 50 ft. interval).

[0039] When a best fit is determined between the frequency response dataof the selected link path and the generic cable mapping in memory, theappropriate correction voltages are applied to the equalizer stage.

[0040] The procedure is carried out for all three colors on three CAT5twisted pairs.

[0041] It is also assumed (to a first approximation) that all CAT5 stylecables of interest for KVM purposes have the same general attenuationcurve to the generic one stored in memory. Of course, the target cableneed not emulate the laboratory cable at common distances, which is anadvantage of the present embodiment. That is, it is enough to know thata length of high quality low loss CAT5 behaves like a shorter length ofpoorer quality cable with a higher loss. In the end, regardless oflength, the appropriate compensation should be applied.

[0042] If this assumption is not sufficiently accurate then the cableequalization system allows for multiple cable mappings to be stored inmemory to accommodate different and more exotic cable types.

[0043] The total time required for the cable equalization routine isapproximately 110 mS, which is essentially undetectable to the user. Theinherent high speed of this approach guarantees minimum disturbance to asecond user that is connected to the same server (and therefore CIP)from another node in the matrix system.

[0044] In an alternative embodiment, the system lends itself to theformulation of a mathematical relationship between the measuredfrequency response data and the applied correction voltages to theequalizer stage. This would eliminate any requirement for cable mappingsand/or look-up tables.

[0045] In another alternative embodiment, video disturbance to all usersis eliminated by sending the equalization tones sequentially during thevertical or frame blanking interval (approximately 500 uS).

[0046] As an added benefit there is no requirement to store linkequalization parameters as cable equalization is performed each time aswitch is made thereby reducing storage requirements for system maps,etc.

[0047] Because all cable pairs that carry color information arecharacterized and equalized separately, there is no requirement for aspecific cable pair pinning assignment. With the appropriate software,deteriorating link conditions due to poor cable interconnects, damagedcable etc. can be detected in advance of a catastrophic failure. Thisinformation can then be used to alert a system administrator so thatpreventative action can be taken.

[0048] Automatic Adaptive Cable Equalization.

[0049] In the example system, one sees that frequency dependentattenuation is automatically corrected up to 1,000 feet of CAT5 stylecable each time a valid KVM path is selected. To do so, the systemincludes:

[0050] (1) An adaptive equalizing filter to synthesize an inverse cableloss curve. The equalizer is based on the summation of an all-passfunction with a n umber of weighted high pass sections. The weighting isimplemented using voltage-controlled amplifiers following each of thehigh pass sections;

[0051] (2) A tone generation block within the Transmitter (i.e. CIP);

[0052] (3) A measurement stage consisting of a source select switch (foreach gun), a Log Amp and an A/D Converter; and

[0053] (4) A software routine to interpret the measured data and applythe appropriate correction values.

[0054]FIG. 2 illustrates an example equalizer circuit employing thecompensation control voltages. FIG. 2 illustrates the red “gun,” butsimilar guns exist for the blue and green components as well. The redcomponent is received on the cable into DC pass through filter 50, theoutput of which is provided to the summer 54. Also provided to thesummer are the outputs of a low band filter 51, medium band filter 52,and high band filter 53. The circuitry of FIG. 2 is a known type ofcircuitry, which is adapted for use with the novel control voltagesemployed by the present embodiment. In essence, the three filters 51-53,provide weighted outputs in order to compensate for video distortion onthe cable. The amount of compensation is determined by the amount ofcontrol voltage applied to each of the filters 51-53.

[0055]FIG. 3 illustrates how the control voltages are obtained. On thesame cable, as previously described, tones are applied to the cable, theoutput of which (i.e., the response of the cable to the tones) areapplied to the log amplifier 55. Ultimately, an array of measured tonevalues are taken and compared to the reference value tables stored inprocessor 57 in order to find a close matching band. This close matchingband is a section of the array that roughly matches the measured valuesprovided into Log Amp 55. The output of the Log Amp 55 is provided to Ato D converter 56, the output of which is provided the processor 57 forthe comparison. The processor performs the matching function accordingto a particular selected match routine and outputs the control voltagesto the D to A converter 58 which produces the V_(L), V_(M), V_(H), andV_(DC) voltages that control the equalizer of FIG. 2.

[0056] One purely example way to match the measured values with themapped values is to choose a mid band frequency to determine the centerpoint of the rough fit table. This rough fit table is determined byscanning the full reference table against the measured value. A deltavalue is calculated as the scan goes across the reference table. Thepoint where the delta value changes polarity is used as the center pointfor the rough fit table. Using this rough fit approach saves processorsearch time in determining the more accurate value. The localized bandis then used as a reference table to zero in on the close match for themeasured values.

[0057] The zeroing in function occurs as follows. From the rough localtable, it is possible to scan a smaller table more thoroughly. Using thefull range of measured values as a seed (i.e., the measured values foreach of the tones used) a scan is performed on the rough table. Thefocus of this scan is to determine the best match with the referencevalues. In one example, in determining the best fit value, the routineplaces more weight on matches with lower frequency tones than it does onthe higher frequency values. The reason behind this is that the mostobjectionable video effects are caused by the result of lower frequencydeterioration (i.e., long streaks or trails of color).

[0058] It is possible to apply other “weighting” depending on theoverall desired effect, and the above example is not limiting of thepresent invention.

[0059] From the scan, a close match value is selected based on one ormore of the following criteria:

[0060] (1) The array value with the most matches across the frequencybands;

[0061] (2) In the case of multiple matches, the array value that has thebest low frequency match; and

[0062] (3) In the case of multiple matches, the first matched value.

[0063] Of course, the artisan can consider many other different types ofschemes for determining a match condition between the measured tonevalues and the reference value array, and the above example is notintended to limit the present invention.

[0064] The resultant match value is output by the processor 57 to the Dto A converter 58 and is then used to set the correction values in theequalizer circuit of FIG. 2. This routine is performed for each of thecolor guns, thus giving a close fit for each of the twisted pairscarrying video.

[0065] If a cable type has special electrical characteristics, areference mapping may be done for that specific cable, which can then beincorporated into the processor code 57. Using this approach allows afully flexible method of determining the most accurate and appropriatecorrection voltage that needs to be applied to a particular length ortype of cable, without knowing the length of the particular cables usedand without imposing inordinate delays on the active compensationsystem.

[0066] The method is further described with respect to FIG. 9. In FIG.9, the request for tone sequences is sent from the AUP to the CIP instep 90. In response, the CIP sends the tones down the cable back to theAUP. If the tones are detected in step 91, the AUP performs measurementcycles for tone N at step 93. The measurements continue at step 93 untilthe maximum number of tones is accomplished at step 94 whereupon the AUPdetermines whether the measured responses are within a window rangeprovided by the stored map. If the measured tones are outside theboundaries at step 96, and the target is too high at step 97, the bestfit is set as the maximum compensation at step 101.

[0067] If the target is outside the boundaries at step 96 and the targetis not too high at step 97, the best fit value is equated to the minimumcompensation at step 100. If on the other hand the measured targets areinside the window boundaries at step 96, the best fit response isdetermined by comparing the measured response with the table response,as previously described, at step 98.

[0068] If the best fit value from step 98 is not valid, the result failsand the method returns to step 90, at step 104. If, the best fit valueis valid at step 99, the best fit value is used to select the correctionvoltage at step 102 via processor 57 of FIG. 3. The correction voltagesare then applied at step 103 to the filters 51-53 of FIG. 2. Thereafter,the process ends at 105.

[0069] If after the tone request is provided by the AUP at step 90 andno tones are detected at step 91 after a time out condition at step 92,the process fails step 104 and returns to the tone request at step 90.

[0070] Automatic Adaptive Cable De-Skew Compensation

[0071] In another embodiment, delay skew on the cable is adaptivelycompensated. Different CAT5 cables have different twist rates tominimize radiation between the pairs. As a result, at the end of arelatively long CAT5 cable, the color component information will arriveat different times for the respective components, yielding spatialseparation on a CRT.

[0072] In the preferred embodiment one determines the slowest pair ofthe CAT5 cable and slows up the other two in order to repair the timecoincidence. This is accomplished by the AUP requesting a tone (thatcould be the same tone used for equalization previously describedabove), for example 750 kilohertz. The tone is sent on all three colorcomponent pairs. Before measurement, a pair select function providesvarious pair selections such as, in the first instance, a comparison ofthe green component versus the red component. (Note that any rounding ofrise times that will inevitably occur as the video travels down the CAT5can be re-squared at the receiving end and will not effect the phasecompensation assuming that both color components are rounded by asimilar amount). Next, a phase comparator gets the output voltage,compared with a predetermined delay, and records a lead or lag togetherwith a value stored for that given pair (green versus red). Next, blueversus red is selected and the process repeated to get a voltagevaluation (and lead/lag) of delay for this color comparison. Next, blueversus green is compared, followed by red versus green (exactly theopposite of the first comparison (green versus red) to allow acancellation of any dc offset of phase compensation circuit due to, forexample, flip flops that may not be perfectly matched, etc.).

[0073] With the above information, the system can determine the relativetime delays per pair. Using that information, the delays can be switchedinto the components themselves.

[0074] In the prior art, delays are provided by lump delays (lcnetworks) which is not very effective. In order to minimize ripples insuch lump delays, there must be lots of stages, requiring lots ofcomponents and lots of space. In this embodiment, printed inductors areplaced spirally on pc boards and capacitors (discrete) added to reach anormalized impedance. Alternatively, the parasitic capacitance of thebilateral CMOS switches used in the switching can be used for thecapacitance in the lc networks, to eliminate or supplement the discretecapacitors.

[0075] Thus, the system can automatically detect and correct forinter-pair delay skew that is inherent in CAT5 style cables. In theexample embodiment, the system is set up with a maximum compensationrange of 128 nS with a 1 nS set-point resolution.

[0076] The de-skew process includes:

[0077] (1) A tone generator block within the Transmitter (i.e., CIP).This can be the same one used for the cable equalization;

[0078] (2) A binary weighted analog delay block for each video path suchas is shown in FIG. 7;

[0079] (3) A measurement stage (such as shown in FIG. 8) consisting of asource select switch for comparing CAT5 signal pairs, a signalconditioning stage using comparators, a charge pump digital phasedetector and an A/D converter; and

[0080] (4) A software routine to interpret the measured data and insertthe appropriate compensating delay values.

[0081] In operation, after the user selects a server from the OSD in theAUP, and a routing protocol opens an appropriate KVM path to theselected server, a tone sequence request is sent to the CIP that isattached to the selected server. The CIP then sends a number of tonesbetween 32.25 KHz and 48 MHz. for cable equalization purposes, asdescribed before. When de-skew is required, an additional 750 kHz toneis sent after the main equalizer tone string. This tone is available onall three color pairs simultaneously.

[0082] In the AUP the source select switch 80 (in the measurement stage)isolates two of the video paths for connection to the measurementcircuitry. High speed comparators 81 “square up” the received tone'sedges (the relative edge to edge timing i.e., delay skew which is whatis of interest remains unchanged) before the digital phase detector 82.

[0083] The digital phase detector 82 is edge triggered with a chargepump at its output. This is followed by an active integrator 83resulting in a linear output voltage for each nS of delay skew at itsinput. The conversion gain is approximately 20 mV/nS. Output polarityindicates lead or lag conditions.

[0084] The integrators output is digitized by the second channel of theA/D converter 84 and stored by processor 85. This process is repeatedfor all other video path combinations, i.e. Green relative to Red, Bluerelative to Red and Blue relative to Green.

[0085] An additional combination of Red relative to Green is used inconjunction with the Green relative to Red above to determine themagnitude of any residual DC offsets within the measurement system.These offsets can then be nulled by the software.

[0086] Once the relative Delay Skews between each of the video pathshave been determined, the processor 85 then controls the delay circuits86 (via switch 77 in FIG. 7) to insert an appropriate delay in to eachof the two quicker paths such that their propagation delay now equalsthat of the slowest path. In FIG. 7, the switch 77 is additive in sensethat it can put one, two, three, four, five, six, all seven, or anycombination thereof of the delay elements 70-776. Because delay elements70-76 are binary, the switch can “add” into the color component (red inthe upper portion of FIG. 7) any amount or delay from 1 ns to 128 nsband on the number and delay value of the delays 70-76 added in.

[0087] Again because all cable pairs that are carrying color informationare characterized and skew compensated for separately there is norequirement for a specific cable pair pinning assignment.

[0088] A number of design approaches were considered in developing thedelay elements used in the De-Skew circuitry. Stripline elements wereoriginally considered in order to preserve bandwidth but were lessworthwhile as the length of copper trace (approximately 66 ft.) requiredto implement 128 nS resulted in un-acceptable losses.

[0089] Commercial lumped delay elements were also considered but werealso less worthwhile as they had an insufficient number of internaldelay sections for a given delay to ensure minimum amplitude ripple andpulse distortion, and their cost was prohibitive.

[0090] The preferred solution consists of discrete LC delay sectionswhere the inductor is implemented as a two turn printed coil in amulti-layer PCB and the capacitor is a discrete surface mount componentcomponent, where

Td={square root}(L×C) and Zo=(L/C)

[0091] Two LC sections per nanosecond of delay are used where L=7.4 nS,C=33 pF and Zo=15R.

[0092] The 128 nS assembly consists of 1 nS, 2 nS, 4 nS, 8 nS, 16 nS, 32nS and 64 nS delay sections 70-76 resulting in a binary weighteddigitally controlled solution.

[0093] The switch element for each delay section consists of two SPDTbi-lateral CMOS switches.

[0094] An alternative and a novel feature of one design is that thesignificant parasitic capacitance associated with the source terminal ofthe CMOS switch becomes an integral part of the required delay linecapacitance at each end of a binary weighted delay section, i.e., it iseffectively “turned out.” This allows the use of low cost CMOS switchelements and yet ensures optimum rise time performance.

[0095] In order to reduce manufacturing costs further it is possible toimplement the delay line capacitors as printed components i.e., both theinductors and capacitors are not printed structures in the PCB copperlayers.

[0096] Method of Reducing Datalink Pair Crosstalk in CAT5 RJ45 Connector

[0097] The following describes a third method of compensation that canbe used independently or in combination with one or both of the priorcompensation methods.

[0098] In bi-directional data links, at extended distances (like 1,000feet and beyond) a high level of video equalization is required. Fiftypoint-to-point right beside the video pairs would not be unusual in sucha case, inducing single ended noise. This embodiment provides deliberatebalancing of the noise in the pair by the imposition of neutralizingcapacitors. This gives some amount of noise on the pairs which will thenbe rejected by the noise rejection circuitry. The result is a removal ofgraininess over long distance communication.

[0099] The physical connection between any Tx and Rx element within thesystem is a 100 Ohm, 4-pair Basic Category 5 (CAT5), Enhanced Category(CAT5e) or Category 6 (CAT6) LTP/FTP cable as specified in TIA/EIA-568-Aor proposed by TIA (CAT5e, CAT6); Class D or Class E as specified inISO/IEC 11801 and the interconnect is a media interface connectorspecified in IEC 60603-7 (commonly referred as RJ45) with pinconnections according to EIA T568B.

[0100] Wire pairings are as follows: PIN NUMBER SIGNAL WIRE COLORS 1Green − Orange/White 2 Green + Orange 3 Blue − Green/White 4 Red + Blue5 Red − Blue/White 6 Blue + Green 7 Data − Brown/White 8 Data + Brown

[0101] The data pair used in this embodiment is a bi-directional EIA 485compliant interface. As a result of the requirement to be compliant withEIA/TIA-568, edge energy from Data−(Pin 7) is capacitively coupled toBlue+(Pin 6) and to Blue−(Pin 3) within the RJ45 housing. As Pin 6 ismuch closer to Pin 7 than Pin 3 the inter-electrode capacitance betweenPin 6 and Pin 7 is therefore greater than the inter-electrodecapacitance between Pin 6 and Pin 3.

[0102] Thus more Data rate edge energy is coupled to Pin 7 than to Pin3. This results in a significant differential noise signal at the inputof the differential Receiver for the Blue gun and appears at its outputalong with the wanted Blue gun signal.

[0103] At extended distances this blue signal is subjected tosignificant high frequency amplification in order to compensate for highfrequency loss in the cable. The induced data rate noise is alsoamplified. This results in “patterning” superimposed on the wantedimage.

[0104] This effect can be minimized if the induced data rate noise onPin 3 is equal in magnitude to the induced data rate noise on Pin 6.This common mode signal is then rejected by the CMRR of the differentialreceiver at the front end.

[0105] This can be easily achieved by connecting a small value“neutralizing” capacitor between 7 and Pin 3.

[0106] The capacitor selected is such that the inter-electrodecapacitance between Pin 3 and Pin 7 is now equal to the inter-electrodecapacitance between Pin 6 and Pin 7.

[0107] Similarly, a second capacitor between Pin 4 and Pin 7 can be usedto minimize data rate noise on the output of the differential receiverfor the Red gun.

[0108] While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1. An analog video switch, comprising: a switch to establish differentconductive paths between a set of workstations receiving analog videosignals and a set of servers sending analog video signals; a memorydevice to store a predetermined table of response characteristics of aspecified conductor path type when said conductor path type receives aset of predetermined frequency tones; a testing circuit in communicationwith the different conductive paths to record a measured response ofsaid conductive paths to said set of predetermined frequency tones; anequalizer circuit to apply compensation signals to components of saidanalog video signals in relation to said measured response of saidconductive paths to said set of predetermined frequency tones.
 2. Amethod as in claim 1, wherein the set of predetermined frequency tonesare selected between the values of 325 KHz and 48 MHz.
 3. A method as inclaim 1, further including a plurality of testing circuits andequalizers, one each per component of said analog video signals.
 4. Amethod as in claim 1, wherein the testing circuit includes at least onecomputer interface pod and at least one user pod.
 5. A method as inclaim 4, wherein the computer interface pod includes circuitry toreceive a tone request from the user pod and respond by applying the setof predetermined frequency tones to the conductive paths.
 6. A method asin claim 5, wherein the user pod issues the tone request and determinessaid measured response.
 7. A method as in claim 1, wherein the equalizerincludes a set of low, middle and high filters independently controlledby said compensation signals.
 8. Analog video de-skew circuitry forvideo compensation of color video transmitted on cables having differenttransmission delays, comprising: a switch to receive on three inputsrespective ones of three color video components, and to select foroutput two of said three color video components, followed by another twoof said color video components, followed by at least one reciprocal ofeither of said two sets of two color video components; square-upcircuitry to substantially square the edges of each of said pairs of twocolor video components; phase detector to detect a phase differencebetween said squared up color video components; an integrator coupled tothe output of the phase detector; a digitizer to digitize the output ofthe integrator; a processor to produce control signals in response tothe output of the integrator; and delay circuits to impose selectivedelays on the three color video components based on the control signals.9. A method as in claim 8, wherein one or more of the square upcircuitry, phase detector, integrator, digitizer, and processor areincluded on a common integrated circuit.
 10. A method as in claim 8,wherein the delay circuits include an additive switch controlling aplurality of binary delay values.
 11. A method of compensating an analogvideo component transmitted over an undetermined cable length,comprising: pre-storing a predetermined table of responsecharacteristics of a specified conductor path type for a number ofincremental conductor path lengths, when said conductor path type ofsaid path lengths receive a set of predetermined frequency tones;pre-storing a set of predetermined compensation values for each of saidresponse characteristic for each of said conductor path lengths; testingsaid undetermined cable length of said conductor path type to record ameasured response of said cable length to said set of predeterminedfrequency tones; comparing said measured response with said pre-storedtable to identify a close match between said measured response valuesand said pre-stored table values; identifying a corresponding ones ofsaid predetermined compensation values; applying said corresponding onesof said predetermined compensation values to an equalizer to compensatesaid analog video component.
 12. A method as in claim 11, wherein theset of predetermined frequency tones are selected between the values of325 KHz and 48 MHz.
 13. A method as in claim 11, further includingreceiving a tone request and responding by applying the set ofpredetermined frequency tones to said cable length.
 14. A method as inclaim 11, further including applying the corresponding ones ofcompensation values to said equalizer as low, middle, and highcompensation values.
 15. A method as in claim 14, further includingapplying the corresponding ones of compensation values to said equalizeras a dc passthrough value.
 16. A cable connector, comprising: a seriesof electrical conductors to electrically couple correspondingelectrically conductive transmission lines, including: (a) first andsecond conductors associated with, respectively, positive and negativered differential analog video signals; (b) third and fourth conductorsassociated with, respectively, positive and negative green differentialanalog video signals; (c) fifth and sixth conductors associated with,respectively, positive and negative blue differential analog videosignals; and (d) a seventh and eighth conductors associated with,respectively, positive and negative digital data signals; a housingcontaining the electrical conductors arranged such that one of saidfifth and sixth conductors is physically closer to the seventh andeighth conductors than the other of said fifth and sixth conductors; anda capacitive element connected between (1) said other of said fifth andsixth conductors and (2) one of said seventh or eighth conductors.
 17. Amethod as in claim 16, wherein the capacitive element is valued tosubstantially neutralize the effect of induced capacitive couplingbetween (1) said physically closer one of said fifth and sixthconductors and (2) said physically closer seventh or eighth conductors.18. A method as in claim 16, wherein the conductors are arranged in aline in the housing in the following order: (1) one of the first,second, third, or fourth conductors, (2) another of the first, second,third, or fourth conductors; (3) one of the fifth or sixth conductors,(4) another of the first, second, third, or fourth conductors; (5)another of the first, second, third, or fourth conductors; (6) the otherof the fifth or sixth conductors; (7) one of the seventh or eighthconductors; (8) the other of the seventh or eighth conductors.
 19. Amethod as in claim 16, wherein the conductors are arranged in a line inthe housing in the following order: (1) fourth conductor, (2) thirdconductor; (3) sixth conductor, (4) second conductor; (5) firstconductor; (6) fifth conductor; (7) eighth conductor; (8) seventhconductor; and the capacitive element is connected between the sixthconductor and the eighth conductor.
 20. A method as in claim 17, whereinthe value is selected to substantially equate the inter-conductorcapacitance between (1) the sixth and eighth conductors and (2) thefifth and eighth conductors.